Pressure switch

ABSTRACT

In a pressure switch, which emits an electrical switching signal as a function of the magnitude of a pressure which is supplied to it, which electrical switching signal assumes a first value when the pressure is less than the threshold value and which assumes a second value when the value is greater than the threshold value, the threshold value is adjusted—assuming that this is provided at all—by hand, for example by moving a cam disk which is coupled to a rotary knob. In order to adjust the threshold value by means of an electrical signal, the pressure is supplied to a pressure sensor, which emits an electrical output signal, which is a measure of the pressure which is supplied to the pressure sensor. The pressure switch is supplied with an electrical input signal which is a measure of the threshold value. The electrical output signal from the pressure sensor and the electrical input signal are supplied to a logic circuit, which emits an electrical switching signal which assumes one of two values as a function of the magnitude of the difference between the electrical signals which are supplied to it. The pressure switch is used generally for signaling on reaching a pressure which can be preselected. The pressure switch is used in particular in automation devices.

[0001] The invention relates to a pressure switch, which emits anelectrical switching signal as a function of the magnitude of a pressurewhich is supplied to it, which electrical switching signal assumes afirst value when the pressure is less than an adjustable thresholdvalue, and assumes a second value when the pressure is greater than thethreshold value.

[0002] A pressure switch such as this is known from U.S. Pat. No.4,388,505. The threshold value, that is to say that pressure at whichthe electrical output signal changes, can be adjusted by rotation of acam disk. A manually adjustable rotary knob is provided in order torotate the cam disk, and has markings for various threshold valuemagnitudes. No provision is made for the threshold value to be changedby signals from a control device.

[0003] The invention is based on the object of providing a pressureswitch of the type mentioned initially, whose threshold value can beadjusted by means of an electrical control signal.

[0004] This object is achieved by the features in the characterizingpart of claim 1. The invention makes it possible to set the thresholdvalue on the basis of an electrical output signal from a higher-levelcontrol device.

[0005] Advantageous developments of the invention are characterized inclaims 2 to 10. In order that pressure signals on which small amplitudepressure fluctuations are superimposed during operation do not initiateswitching processes caused by the pressure fluctuations, the logiccircuit for the electrical input signal and for the electrical outputsignal from the pressure sensor is also supplied with a correctionsignal, which shifts the pressure at which the switching process takesplace with respect to the threshold value which is predetermined by theelectrical input signal. If the switching process is intended to beinitiated only at a pressure which is greater than the threshold value,the correction signal acts as in increase in the threshold value. If, onthe other hand, the switching process is in fact intended to beinitiated at a pressure which is less than the threshold value, thecorrection signal acts as a reduction in the threshold value. In thiscase, the correction signal may be constant or may depend on themagnitude of the pressure to be monitored. If the logic circuit issupplied with a second correction signal in addition to the firstcorrection signal, two different switching points can be provided. It isthus possible to initiate a switching process not only at a pressurewhich is greater than the threshold value, but also at a pressure whichis less than the threshold value. It is thus possible, for example whenmonitoring the operating of a clamping system by means of a pressureswitch, to at the same time monitor whether a minimum pressure, which isrequired for secure clamping of a work piece, exists, and whether thepressure is less than a maximum pressure, which would lead to damage tothe work piece. A constant correction signal can advantageously bederived from a supply voltage which is provided to the pressure switchin order to supply the pressure sensor and the logic circuit.

[0006] A pressure switch such as this may advantageously be used inorder to monitor the operating pressure of a hydraulic load in a machinetool, in particular in clamping systems with hydraulic cylinders foroperating a chuck or the tail spindle of a tailstock. In this case, thenominal value signal which is supplied to the drive electronics for thevalve from a higher-level machine controller is also supplied as anelectrical input signal to the pressure switch. The electrical switchingsignal from the pressure switch is supplied to the higher-level machinecontroller. In a corresponding way, in the case of a pressure switchwith two electrical switching signals, both switching signals aresupplied to the machine controller. This pressure monitoring arrangementhas the advantage that the pressure monitoring can be carried outindependently of the valve control. However, in this case, the switchingthreshold of the pressure switch is also driven by the control signalfor valve control. The higher-level machine controller evaluatesswitching signals from the pressure switch and initiates safety-relevantmeasures if the operating pressure exceeds the predetermined limits inone direction or the other direction.

[0007] The invention will be explained in more detail in the followingtext, together with its further details, on the basis of exemplaryembodiments which are illustrated in the drawings, in which:

[0008]FIG. 1 shows the block diagram of a pressure switch having oneswitching output,

[0009]FIG. 2 shows the block diagram of a pressure switch having twoelectrical switching outputs, and

[0010]FIG. 3 shows an arrangement for monitoring the operating pressurein a hydraulic clamping system, having one pressure switch which has twoswitching outputs.

[0011]FIG. 1 shows the block diagram of a pressure switch according tothe invention, having one electrical switching output. The pressureswitch is annotated by the reference symbol 10. The pressure switch 10is supplied with a pressure pe, which is to be monitored, as the inputvariable. A pressure sensor 11 converts the pressure pe to a voltageua1. The magnitude of the voltage ua1 is a measure of the pressure pe.In the exemplary embodiment under consideration here, the voltage ua1 isassumed to be proportional to the pressure pe. The output voltage ua1from the pressure sensor 11 is supplied as an input signal to a logiccircuit 12. Other input signals to the logic circuit 12 are a voltageues and an voltage uk1. The voltage ues is a measure of the thresholdvalue of the pressure switch 10, which is referred to by ps in thefollowing text. The voltage uk1 is the product of the voltage ues and afactor k1. The logic circuit 12 comprises an addition element 13 and aswitching element 14, which is downstream from it. The output signalfrom the addition element 13 is a voltage ud1. This voltage is suppliedas an input signal to the switching element 14. The output signal fromthe switching element 14 is a voltage ua2. The voltage ua2 assumes afirst value ua20 when the voltage ud1 which is supplied to the switchingelement 14 is negative, and it assumes a second value ua21 when thevoltage ud1 which is supplied to the switching element 14 is positive.The voltage ua2 of the switching element 14 is the output signal fromthe logic circuit 12, and also the output signal from the pressureswitch 10. The pressure switch 10 is supplied with a voltage Uv, whichis used as a supply voltage for the pressure sensor 11 and for the logiccircuit 12. The voltage Uv is either a constant voltage, or may varyabout a nominal value within a tolerance band. First of all, it isassumed that the factor k1 is equal to zero. The voltage uk1 is thusalso equal to zero. In this case, the addition element 13 forms only thedifference between the voltage ua1 and the voltage ues, that is to saythe voltage ud1 which is supplied to the switching element 14 is definedby the relationship ud1=ua1−ues. The voltage ud1 is negative when thepressure pe which is supplied to the pressure sensor 11 is less than thethreshold value ps. In this case, the voltage ua1 is less than thevoltage ues. As stated above, the voltage ua2 assumes the value ua20 fornegative values of ud1. The voltage ud1 is, on the other hand, positivewhen the pressure pe which is applied to the pressure sensor 11 isgreater than the threshold value ps. In this case, the voltage ua1 isgreater than the voltage ues, and the voltage ud1 is positive. Thevoltage ua2 assumes the value ua21. The threshold value ps for thepressure pe which is to be monitored can be varied easily by varying thevoltage ues. Since there is a fixed relationship between the pressure peand the output voltage ua1 from the pressure sensor 11, each pressurevalue pe can be associated with a corresponding value of the voltageues. In this case, there is no need to set the threshold value on thepressure switch itself, and the threshold value can be predetermined byremote control, for example on a higher-level control device.

[0012] In situations in which small amplitude pressure fluctuations aresuperimposed on the pressure to be monitored during operation, theinvention allows the threshold value ps to be set on the basis of themean value, over time, of the pressure to be monitored. However, inorder to avoid initiating a switching process whenever the mean value isundershot, the addition element 13 is also supplied with the voltage uk1with a negative mathematical sign, as a correction signal. This is doneby choosing the factor k1 to have a value greater than zero. The voltageuk1 acts like a reduction in the voltage ues. Since the voltage uk1 isproportional to the voltage ues, its correction effect is greater forhigh values of the voltage ues than for small values. If such arelationship between the correction voltage uk1 and the voltage ues isundesirable, it is also possible—as described further below withreference to FIG. 2—to supply the addition element 13 with a constantcorrection voltage. If the correction voltage uk1 is supplied to theaddition element 13 with a positive mathematical sign rather than with anegative mathematical sign—as illustrated in FIG. 1—the switching pointof the pressure switch 10 is shifted to values which are greater thanthe threshold value which corresponds to the voltage ues. A correctionsuch as this makes it possible to monitor a pressure on which smallamplitude pressure fluctuations are superimposed for overshoots, that isto say the switching process of the pressure switch 10 is initiated onlywhen the instantaneous value of the pressure pe exceeds a value which isgreater than the mean value, over time, of the pressure to be monitoredby a value that is governed by the factor k1.

[0013] Based on FIG. 1, FIG. 2 shows a pressure switch 20 having twoswitching outputs. As in FIG. 1, the pressure pe to be monitored issupplied to the pressure sensor 11. The pressure sensor 11 converts thepressure pe to the voltage ua1. The voltage ua1, the voltage ues and thevoltage uk1 are supplied as input signals to a logic circuit 21. Avoltage uk2 is also supplied, as a second correction signal, to thelogic circuit 21. The voltage uk2 is a constant voltage which—forexample with the aid of a voltage regulator 22—is derived from thevoltage Uv which supplies auxiliary power to the pressure switch 20. Thelogic circuit 21 has three addition elements 23, 24 and 25, of which theaddition elements 23 and 24 correspond, in terms of their effect, to theaddition element 13 in FIG. 1. The addition element 23 uses the voltagesues and ua1 to form a voltage ud0, which is supplied not only to theaddition element 24 but also to the addition element 25. The additionelement 24 uses the voltage ud0 and the voltage uk1 to form the voltageud1. This voltage is supplied—as in FIG. 1—to the switching element 14,whose output voltage ua2, with the values ua20 or ua21, is the firstoutput signal from the pressure switch 20. As long as the pressure pewhich is to be monitored is less than the threshold value ps reduced bythe first correction value, the voltage ud1 is negative, and the outputvoltage ua2 assumes the value ua20. If the pressure pe which is tomonitored is greater than the threshold value ps reduced by the firstcorrection value, the voltage ud1 is positive, and the output voltageua2 assumes the value ua21. The addition element 25 uses the voltagesud0 and the voltage uk2 to form a voltage ud2. The voltage ud2 issupplied to a further switching element 26, which is designed in thesame way as the switching element 14. The output signal from theswitching element 36 is a voltage ua3, which assumes the value ua30 whenthe voltage ud2 is negative, and assumes the value ua31 when the voltageud2 is positive. The voltage ua3, with the values ua30 or ua31, is theother output signal from the pressure switch 20. As long as the pressurepe which is to be monitored is less than the threshold value psincreased by the second correction value, the voltage ud2 is negative,and the output voltage ua3 assumes the value ua30. If the pressure pewhich is to be monitored is greater than the threshold value psincreased by the second correction value, the voltage ud2 is positive,and the output voltage ua3 assumes the value ua31. A single thresholdvalue ps in the form of the voltage ues thus makes it possible tomonitor the pressure pe both for overshooting a pressure value ps−Δp1,which is reduced below the threshold value ps, and for exceeding apressure value ps+Δp2, which is increased above the threshold value ps.In this case, the pressure Δp1 corresponds to the voltage uk1, and thepressure Δp2 corresponds to the voltage uk2.

[0014] In FIG. 2, the voltage uk2 is a constant voltage. In contrast tothis, however, the voltage uk2 may, when required, also be a function ofthe voltage ues—as explained, by way of example, on the basis of thevoltage uk1.

[0015]FIG. 3 shows the use of the pressure switch 20, as illustrated inFIG. 2, for monitoring the operating pressure of a hydraulic load for amachine tool. A hydraulic load in the form of a double-acting cylinder30 operates a chuck 32, which is illustrated only symbolically here, viaits piston rod 31. Instead of a chuck, this may also be the tail spindleof a tailstock, for example, which is pressed by the piston rod 31against a work piece, which is not illustrated in FIG. 3. The chambersof the cylinder 30 are provided with the reference symbols 33 and 34. Achangeover valve 35 is connected to the lines 37 and 38, which lead froma switching valve 36 to the chambers 33 and 34 of the cylinder 30. Theoperating pressure of the clamping apparatus is the higher of thepressures in the chambers 33 and 34. This pressure is supplied to thepressure switch 20, via the changeover valve 35, as the pressure pewhich is to be monitored. The switching valve 36 connects one of thechambers 33 and 34 of the cylinder 30 to the output connection of apilot-controlled pressure regulating valve 39, and connects therespective other chamber 34 or 33 of the cylinder 30 to a tank 40,depending on its position. The pressure regulating valve 39 is connectedvia a further connection to a pump 42, which is driven by a motor 41,and via a third connection to the tank 40. The pump 42 supplies thepressure medium to the pressure regulating valve 39.

[0016] A higher-level machine controller 43 supplies drive electronics44 for the pressure reducing valve 39, via a line 45, with a voltage uvsas a nominal value signal for the operating pressure to be set for theclamping apparatus. This voltage is at the same time supplied to thepressure sensor 20 as an electrical input signal ues which—as statedabove—is a measure of the threshold value. The pressure regulating valve39 regulates the operating pressure pe of the pressure medium which issupplied to the cylinder 30, in accordance with a control signal whichis formed by the drive electronics 44. This control signal is suppliedto the pressure regulating valve 39 via a line 48. The output pressurepe from the pressure regulating valve 39 is adjusted by means of a DCdrive with closed-loop position control. The position feedback to thedrive electronics 44 is provided via a line 49. The position controlloop is closed in the drive electronics 44, and there is thus no needfor any position feedback to the machine controller 43. The machinecontroller 43 may be supplied, for example, with status messages fromthe drive electronics 44 via a line 50, which is arranged between thedrive electronics 44 and the machine controller 43. The switching valve36 is operated directly by the machine controller 43. Lines 51 and 52are provided for this purpose, and are routed from the machinecontroller 43 to the switching valve 36.

[0017] The pressure switch 20 monitors the operating pressureindependently of the operation of the pressure regulating valve 39,which is adjusted via the drive electronics 44. However, the nominalvalue signal for valve control and the threshold value of the pressureswitch have the same origin. As stated above, the voltage uvs issupplied as an electrical input signal ues to the pressure switch 20.The output signals ua2 and ua3 from the pressure switch 20 are suppliedas binary input signals to the machine controller 43. The machinecontroller 43 evaluates the output signals ua2 and ua3 from the pressureswitch 20. If the permissible ranges, which are governed by the voltagesuk1 and uk2 from the pressure switch 20, are exceeded, the machinecontroller 43 initiates corrective measures, which change the machinetool to a safe state. In this case, it is advantageous for the drive forthe pressure regulating valve 39 to operate with automatic locking, thatis to say, if the electrical control signal for the pressure regulatingvalve 39 fails, for example due to a discontinuity in the line 48, thelast pressure setting is maintained.

[0018] The use of the pressure switch according to the invention is notrestricted to the examples mentioned above. The pressure switchaccording to the invention may be used wherever pressure monitoring isrequired.

1. A pressure switch, which emits an electrical switching signal as afunction of the magnitude of a pressure which is supplied to it, whichelectrical switching signal assumes a first value when the pressure isless than an adjustable threshold value, and assumes a second value whenthe pressure is greater than the threshold value, characterized by thefact that the pressure (pe) is supplied to a pressure sensor (11) whichemits an electrical output signal (ua1) which is a measure of thepressure (pe), that the pressure switch (10, 20) is supplied with anelectrical input signal (ues) which is a measure of the threshold value(ps), and that the electrical output signal (ua1) from the pressuresensor (11) and the electrical input signal (ues) are supplied to alogic circuit (12; 21), which then emits an electrical switching signal(ua2) which assumes one of two values (ua20, ua21) as a function of themagnitude of the difference between the electrical signals (ua1, ues)which are supplied to it.
 2. The pressure switch as claimed in claim 1,characterized by the fact that the logic circuit (12; 21) is suppliedwith an electrical correction signal (uk1) which shifts the pressure atwhich the switching process takes place with respect to the thresholdvalue (ps) which is predetermined by the electrical input signal (ues).3. The pressure switch as claimed in claim 2, characterized by the factthat the correction signal (uk1) increases the pressure at which theswitching process takes place with respect to the threshold value (ps)which is predetermined by the electrical input signal (ues).
 4. Thepressure switch as claimed in claim 2, characterized by the fact thatthe correction signal (uk1) reduces the pressure at which the switchingprocess takes place with respect to the threshold value (ps) which ispredetermined by the electrical input signal (ues).
 5. The pressureswitch as claimed in claim 3 or claim 4, characterized by the fact thatthe correction signal (uk1) is constant.
 6. The pressure switch asclaimed in claim 3 or claim 4, characterized by the fact that thecorrection signal (uk1) is a function of the electrical input signal(ues).
 7. The pressure switch as claimed in claim 5 or claim 6,characterized by the fact that the correction signal (uk1) isproportional to the electrical input signal (ues).
 8. The pressureswitch as claimed in one of claims 4 to 7, characterized by the factthat the logic circuit (21) is supplied with a second correction signal(uk2), which increases the pressure at which the switching process takesplace with respect to the threshold value (ps) of the pressure switch(20) which is predetermined by the electrical inputs (ues), and that thelogic circuit (21) forms a second electrical switching signal (ua3) fromthe electrical output signal (ua1) from the pressure sensor (11), theelectrical input signal (ues) and the second correction signal (uk2). 9.The pressure switch as claimed in claim 8, characterized in that thesecond electrical switching signal (ua3) changes from a first value(ua30) to a second value (ua31) when the electrical output signal (ua1)from the pressure sensor (11) is greater than the sum of the electricalinput signal (ues) and the second correction signal (uk2).
 10. Thepressure switch as claimed in claim 9, characterized in that the secondcorrection signal (uk2) is a constant voltage, which is derived from asupply voltage (Uv) which is supplied to the pressure switch (20). 11.Use of a pressure switch as claimed in one of the preceding claims formonitoring the operating pressure of a hydraulic load for a machinetool, in particular in clamping systems with a hydraulic cylinder foroperating a check or the tail spindle of a tailstock.
 12. Arrangementfor monitoring the operating pressure of a hydraulic load (30) for amachine tool, in particular in clamping systems with a hydrauliccylinder for operating a chuck or the tail spindle of a tailstock,having an electrically operated valve (39) for controlling the pressureof the pressure medium which is supplied to the hydraulic load (30),having drive electronics (44) for the valve (39), having a machinecontroller (43) at a higher level than those, and having a pressureswitch (10; 20) as claimed in one of claims 1 to 10, with the pressureswitch (10; 20) being connected between the valve (39) and the hydraulicload (30), the nominal value signal (uvs) which is supplied to the driveelectronics (44) for the valve (39) from the higher-level machinecontroller (43) being supplied to the pressure switch (10; 20) as theelectrical input signal (ues), and the electrical switching signal (ua2)from the pressure switch (10; 20) being supplied to the high-levelmachine controller (43).
 13. The arrangement as claimed in claim 12having a pressure switch as claimed in one of claims 8 to 10,characterized by the fact that the second electrical switching signal(ua3) from the pressure switch (20) is supplied to the higher-levelmachine controller (43).